Bill, NI Tech support suggested I save my project in LV60 format, recompile, and see if the error still occurs. I did this. However, I could not export the .bld file from LV61 to 60, so I had to re-create it. No error occurred when I compiled this (with installer) in LV60. I then loaded this project back into LV61 and compiled it - also, no error! So I have no idea what caused this. However, I did ask NI for a wish list item - to add more error info in App Builder, so that one knows what input parameter or VI caused the problem. (With 300 Vis is a project, it is a daunting task to find the probelm otherwise).